The cross-referenced German published application discloses a signal regenerating device as described above. The apparatus disclosed therein is suitable for transmission of either analog or digital signals. Signals can be transmitted at high speeds and with very small amplitudes so that the apparatus is particularly useful for linear transmission of measuring signals. In this equipment the oscillations from a high frequency generator on the secondary side are inductively coupled to a resonant circuit on the primary side. A modulation circuit on the primary side modulates the oscillations in dependence on the communication signals received at the input of the apparatus. The modulated high frequency signals are inductively coupled to a demodulation circuit on the secondary side. The demodulation circuit removes the high frequency components and its output is an exact reproduction of the input signals. Depending upon the type of construction and the sensitivity of the modulation circuit, signals of variable amplitude may be regenerated so that either analog or digital signals may be processed. The shape of the output signals of the regenerating apparatus is exactly that of the input signals since the amplitude of the input signals determines the amplitude of the high frequency oscillations coupled back to the secondary side.
In this known apparatus the high frequency generator continually furnishes high frequency oscillations and these high frequency oscillations are modulated by the incoming signals. This causes an undesired DC component to appear after demodulation. This DC component must be removed if the output signals are to be an exact reproduction of the input signals. In the known apparatus the removal of the DC component was accomplished by an output amplifier which had a predetermined threshold voltage. However, variations of the undesired DC component can occur due to temperature-dependent changes in the output voltage and/or frequency of the high frequency generator. Such varying effects cannot be removed by a constant threshold of an output amplifier.